text: Anisotropic displacement parameters are often useful for visualizing matter. From (15), we may define ellipsoids of constant probability for which , where  is some constant. Such "vibration ellipsoids" have been used to illustrate crystal structures. Alternatively, mean square displacement surfaces along  may be defined by . See the external links "Gallery of ray-traced ORTEPs", "2005 paper by Rowsell et al'.", and "2009 paper by Korostelev and Noller" for more images. Anisotropic displacement parameters are also refined in programs (e.g. GSAS-II) to resolve scattering spectra during Rietveld refinement.